Structural Questions for Porcelain Floor

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Old 05-05-15, 07:17 AM
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Structural Questions for Porcelain Floor

We are renovating our home built in mid-60s and want to tile a large area on the main floor. The original floor had square red quarry tile in a thick mud bed that must have been about 1 1/4" thick or more and weighed a ton. The mud, lath, and roofing paper were all removed. The supporting base for this was plywood nailed between the joists on 1x2s that run the length of the joists and was left in place. The Joists are Douglas Fir 2x9s, 16" on center. Underneath the joists there are 1x3s, 12" on center, nailed perpendicular to the joists to which ceiling tiles are stapled. The unsupported span of the joists is 160".

A builder who was helping me screwed and glued down 5/8" ply everywhere. Much of the center section of the floor sagged approximately 3/8", some a bit more (amazingly we had not had cracked tiles or grout, just an uneven floor) so I put down 3/8" plywood sheathing everywhere, save for the last three feet or so near the outside walls as this area was higher already and I would like the floor level to match the old as closely as possible. The floor seems solid, but I am not an expert.

My plan is to level everything using Ultraflex 2 and put down Ditra XL, followed by Kerabond and 12x24" tiles. Originally we had wanted to go with stone, but after reading some of these threads and the Ditra manual, I realized that was not a good idea so we are going with a porcelain.

My question is: Does this plan have a good chance of working given my underlying structure? I have read comments about adding to the joists to beef them up, but am wondering whether what I have already is sufficient or if I need to tear out the drop ceiling below and add to it. I have not been able to find a table for joist spans, and am not sure how much the other material might be helping.
 

Last edited by MikeRe; 05-05-15 at 07:41 AM.
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Old 05-05-15, 08:14 AM
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Welcome to the forums and thank you for providing as much information as you have up front. Jaz will be along shortly and provide you with his take. One thing he will ask is if you happened to note the grade species of the douglas fir? It works heavily into the span tables he uses. However, as you have already noted, there is sag in your floor. Whether it is from the weight of the concrete that you removed or from over spanned joists is to be determined. What you need to determine is movement. Try a simple test - take a large pitcher of water and place it near the center of the room. Then with a fairly aggressive walk, skip and/or jump, walk around the pitcher of water and look for ripples in the water. That will tell you how much movement you have. If you have a 360 degree self leveling laser beam, you can place that in the middle and walk around. For that you look for the beam to wave on the walls as you walk around.

Personally, I so wish that you had put down 3/4" instead of 3/8" plywood. And did you use underlayment grade plywood or sheathing?
 
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Old 05-05-15, 09:13 AM
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Thanks for the reply. Here is a picture of the stamp on the joists.Name:  IMG_0544.jpg
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Am not sure if we failed the water pitcher test or not. I used a very full glass pitcher. Walking briskly around, I got small riples. Hopping, running, stomping, more pronounced ripples, but no sloshing or spilling.

From reading on this site, I understand it would be better to have more plywood; unfortunately a lot of mistakes were made along the way and we now have kitchen cabinets in with only 3/4" total left to work with (they are sitting on 3/4" ply on top of the underlayment). I think most of the 3/8" plywood is rated for underlayment but I ran out and had to get four sheets at HD, which only had the sheathing grade. Of course, they told me it would work fine.

I was hoping that by using the Ditra XL that the extra thickness of that product might compensate somewhat for our puny plywood. They have pretty limited info, but it does seem to be a significant upgrade to the regular Ditra.
 
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Old 05-05-15, 02:55 PM
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Construction grade per your stamp is somewhere between a #2 grade and a #3 grade and will most likely come back as over spanned for tile or at least stretching the maximum limits. Do you remember seeing any blocking between the joists? Solid blocking will add some stiffness. Unfortunately, the ditra is an uncoupling membrane which provides for lateral slip between the ply and ditra. It also makes for a good surface with which to attach your tile. Tile should not be attached directly to plywood. The ditra and 3/8" ply will not add any real structural integrity to the subfloor system. You have deflection from the joists span, you also have deflection of the material between the joists to take into account. Which direction were you going to lay your tile?
 
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Old 05-05-15, 03:43 PM
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Thanks for your responses. There is metal cross bracing in the middle of the span; am not sure if that is as good as 2x9 blocking, but it is something. Unfortunately, the 1x2s that support the plywood that is between the joists, about 1/4" below the surface, end on either side of these cross braces and the plywood is notched. Do you think that nailing 2x4s to the middle of the joists, through the open area of the cross bracing, would do enough good to warrant opening up the ceiling?

FWIW, I repeated the water test, this time moving the pitcher around to various points in the room. In the kitchen area, which has a smaller unsupported span of under 89", I had to stomp on the floor next to the pitcher to get even the slightest ripple. This compares to the middle of the great room, which has the maximum unsupported span where I can cause slight ripples from several feet away by skipping and stomping. For comparison, I also tried the test on the subfloor of an addition we put on that will also be tiled. That area was built with 3/4" Advantec flooring on engineered joists that only run about eight feet and does not having anything else on it. I tried and tried and could not make a ripple.

I was planning on running the tile lengthwise in the room, or perpendicular to the joists; however, if there is a reason that is not advisable, I would run them any way that will work best, including on a diagonal if need be.
 
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Old 05-06-15, 12:29 PM
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Hi guys,

I took yesterday off cuz I worked the election poles and so it was a 16 hour day before I returned to the home office.

Okay, so, you had a good old fashioned mud job and now it's gone. The subfloor was installed between the joists and the top is about " below the top of the joists to accommodate the deck mud. Good, that's the way it was supposed to be done.

You installed ⅝" subfloor grade plywood (hopefully), then a layer of some ⅜" ply. That was a mistake but................oh well.

I think your joists spanning 13'4" are over spanned and need help. You said the floor may have sagged ⅜", but remember that most joists have a small crown and the crown goes facing up when installed. So they may have sagged much more. If the joists had been #2, I wouldn't span more than 12'10". if they were #3, 9'10". So the right answer is someplace between those spans. Your house was built to meet min. specs at 40/10 load, not 50/20 which is what I used.

Might be a good time to get rid of those old ceiling tiles in the basement, then jack up the joists and sister them.

Forget the water test unless there's a dial in there some place. That might give an experienced person a "sense" of stiffness, but nothing you can depend on.

Jaz
 
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Old 05-06-15, 02:40 PM
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Thanks for your reply; sounds like yesterday was a long day.

Not crazy about tearing up the ceiling, only because of the extra work and the fact that it was one area of the house that hasn't been torn up by construction yet. Oh well.

Just to confirm, my joists are 50 year old Doug fir with a nominal width of 9". There are 1x2s on both sides with plywood nailed to those, and more plywood nailed on top of the joists where none was before. We also took a lot of weight off. I am also using Ditra XL. The company claims that XL makes it possible to have wider distance between joists and less ply on top and was really hoping that would make a difference. Guess what you are saying though is that the joists still need to be properly sized for the unsupported run? Could you expand a bit on the load calc you mentioned?

If I jack up the joists and sister in new wood, would 2x4s work, or do I need to go with something larger? The reason I ask is that there are not only the cross bracing brackets that would need to be removed for larger lumber, there are also wires in the way in some places. Have also heard of putting the 2x4 on the bottom but not sure how I would do that if I jack up the joists. Finally, would I need to sister the entire span, or just a portion of it?

Thanks so much for you help, we really appreciate it and are very glad to have found this forum!
 
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Old 05-06-15, 04:50 PM
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Mike,

I am not a structural engineer but I am giving you advice we have found to be safe for tile work. I am not saying that if you do not meet the min. requirements in the industry that your floor will fail for sure. We are talking "best practice", but wood joists can take lots of stress before they fail completely.

Normal code for residential is for the joists and subfloor to meet L360 deflection. That is = 1" deflection in 360" span or any fraction there of. Basic flooring code for the load in any room other than sleeping rooms is L360 at 40/10, live/dead lbs. per sq. ft. Live is people and movable objects, dead is the weight of the entire floor component above the joists. So, 10# isn't much and neither is 20#.

In comparison stone tiles require min. L720. The good news is that the deflection of the joists is more forgiving than the deflection between the joists, because of increased curvature in the shorter distance. Will a floor be ok at L360 but fail at L359? Of course there is no way to know for sure. We just use these numbers for design/building reasons, under the right circumstances a floor could fail with better numbers.

If you plan to sister; you can use narrower lumber such as 4", 6" or 8" and should try to sister at least the mid ⅔ of the span. Some have even used " ply or OSB laminated and screwed to the joists. Wires? They can be cut and re spliced in new boxes on either side. You could install a beam, or build a supporting wall to reduce the span a few ft. Anything you do like that will help.

Your joists should measure 1.5 or 1 ⅝" x 9.25" or very close to that. The 1x2 are not doing much if anything at all, especially near the top of the joists. The furring strips for the ceiling tiles and the ceiling tiles add load, so that is a negative factor.

Jaz
 
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Old 05-06-15, 05:41 PM
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Thanks, Jaz, forgot about the plywood being recessed by 1/4", so about 9 1/4" would be right.

Have been reading some other blogs and engineers have been saying that a 2x4 on the bottom of the joist is much more effective in stiffening the structure than ones sistered on both sides. I also saw the This Old House guys recommending the ply glued and screwed to the sides to help prevent deflection. I may do the first or a combination of these since if I am taking the ceiling down the difference in lost head room will be less than an inch since I have 1x3s there now.

The reason I mentioned the 1x3s and the ceiling tiles is based both on the idea that a structural member is most effective at the maximum point of deflection, which would be the lowest point on the joist, and the idea that while the 1x3s are not stiffening the length of the joists, they would help transmit the load at the max point of deflection to the other joists. In another blog there was actually the assertion by some that blocking or bracing is really only needed if the area below is not finished. I was really surprised that drywall would help that much to minimize deflection and just wondered if the system I had would do anything in that regards.
 
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Old 05-06-15, 06:57 PM
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I don't see how drywall or 1x3 furring strips installed perpendicular to the joists could possibly stiffen the joists.

I'd rather have a sister on the side of the joists than one fastened flat to the underside of the joists. The strength axis of the lumber doesn't make sense doing it the other way. But of course, the sister should be at the bottom of the joist, and I recommend to jack it up slightly before it's attached so it'll actually share the weight. You won't find that written anywhere but here. I think I'm right.

The X bridging is there to help share/spread the weight/deflection too.

Jaz
 
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Old 05-07-15, 09:36 AM
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Am not an engineer either but jacking up the joists a little makes sense. Thanks for that.

If you go back and look at what I wrote previously, I noted that the 1x3s and ceiling tiles would not add stiffness, but would help to spread out the deflection throughout the structure.

A structural engineer on another site ran through a quick calculation for a 2x8 joist and found that while adding two 2x4s on the sides flush to the bottom of the joist would increase the moment of inertia by 60%, using only one 2x4 horizontally on the bottom of the 2x8 would increase the moment of inertia by 250%. Apparently there is a reason that I beams are shaped the way they are.

He did state, however, that the difference would decrease as the size of the joists increased, so there should be less difference between the two methods for my 2x10s.

Additionally, the neutral point of the joist is actually in the middle, according to his note, so my early idea of placing a 2x4 down the middle apparently would have been the least effective method.

Was hoping to start tiling next week, but not sure if I will be able to have the drywall guys out to redo the ceiling I am tearing out before then. Do you think we should hold off on tiling the floor above until the ceiling is put up below? Just wondering if the force from the screw guns would be enough to cause problems.
 
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Old 05-07-15, 06:27 PM
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Mike, I certainly understand the principle of an I-beam and agree it's superior to any other. I just think it works better on paper than in a case where we retrofit an existing joist. But, if you can laminate and fasten well enough, it'll do the job. I believe loading the joist is essential and hope you're able to do that too.

Back to the comparison for a minute though. I think comparing the 2x4 flat to the bottom I-beam to 2x4 on the side is not a fair comparison. I'd recommend min 2x6" or 2x8" if possible.

And yes, the neutral point is a short span in the middle where either side would bend under load. That's why we recommend sistering at least the mid ⅔ of the span if you can't run the entire length.

There shouldn't be any stress to the joists by fastening drywall screws below the room. But if it were me, I would wait if there was no deadline.

Jaz
 
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Old 05-08-15, 03:49 AM
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One option we have not really discussed yet would be to redesign the layout of the downstairs room. All we know at this point is that it is finished or at least has a finished ceiling. One thing that can be done is partition off a section with a wall, as in building a closet on one side perpendicular to the joists to shorten the span without tearing everything out to sister joists, monkey with cross bracing, electrical and plumbing issues. Jack the floor up, install a tight fitting 2x4 wall and set the floor back down on it. A 2' deep closet or storage area would put your span more in spec.
 
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